1. Technical Field
This invention relates to optical microscopes in which a small aperture, typically smaller than an optical wavelength, is positioned within the optical near field of a specimen, i.e., within a distance of less than about an optical wavelength of the specimen, and the aperture is scanned in raster fashion over the surface of the specimen to produce a space-varying optical signal that is detected and reconstructed to produce an image having extremely high resolution. The invention further relates to methods for using such microscopes to inspect workpieces during their manufacture.
2. Art Background
Microscopes employing conventional optical imaging systems cannot resolve features substantially smaller than about one-half an optical wavelength. That is, when both the entrance pupil of the microscope objective and its distance from the specimen are substantially larger than a wavelength, diffraction effects limit the smallest resolvable separation between a pair of object points to 0.5 .lambda./N.A., where .lambda. is the optical wavelength and N.A. denotes the numerical aperture of the objective which, as a practical matter, is limited to values of about 1.4 or less. (F. A. Jenkins and H. E. White, Fundamentals of Optics, pp. 306-308 (3rd ed. 1957).
Near-field microscopy has been successful in resolving features much smaller than those resolvable with conventional optical microscopy. One such microscope is disclosed in U.S. Pat. No. 5,288,999 to Betzig et al., the disclosure of which is incorporated by reference. The microscope described in Betzig et al. is described with reference to FIG. 1. The optical system includes a light source 10, a probe 20, displacement means 30 for displacing the probe relative to an object 40 disposed, for example on a stage 50, adjacent the probe tip 60. The optical system further comprises a light source 10 optically coupled to a probe 20. A single-mode fiber 70 can be used as the optical coupling. Light source 10 is, for example, a laser. Light from source 10 is injected into the optical fiber by way, e.g., of a single-mode coupler 80, which includes a microscope objective 90 and a fiber positioner 100. A mode stripper 110 is also optionally included in order to insure that only the single mode in the core is propagated to the probe, and the other modes in the cladding. The displacement means 30 may, for example, be a piezoelectric tube adapted for moving the probe vertically as well as in two orthogonal lateral dimensions. Alternatively, the displacement means may be mechanical or piezoelectric means for moving the stage rather than the probe, or some combination of stage-displacement and probe-displacement.
Near-field microscopy has been used to detect very subtle changes in the surface of a sample. For example, near-field optical microscopy has been used to inspect latent images in energy-sensitive resist material used in lithographic processes for semiconductor device fabrication. The latent image is the image introduced into the energy-sensitive resist material by exposing the material to patterned radiation. The results of the inspection have been used to control the process parameters in lithographic processes for device fabrication. Such a technique is described in Marchman, H., et al., "Near Field Optical Latent Imaging With the Photon Tunneling Microscope," Applied Physics Letters, Vol. 66 (24), pp 3269-3271 (1995) In the technique, near-field imaging is used to inspect the latent image introduced into the resist via a patternwise exposure to radiation. A spatially resolved image of the latent features is obtained and the spatially resolved image is compared with the desired pattern. The spatially resolved image is a picture of the intensity of the light either reflected from or transmitted through a point on the sample relative to the intensity of the light either reflected from or transmitted through other points on the sample. The variations in intensity are cause by variations in the index of refraction and the topography of the resist material as a result of the patternwise exposure to radiation. These dark and light areas provide an image of a pattern introduced into the energy sensitive resist material.
Because latent image metrology is a non-destructive technique that provides information in real time, improvements in the techniques which provide even greater information about the latent image are desired.